The belt tensioner serves to act upon a belt spool of a belt retractor associated with, in the winding direction of the safety belt, in order to eliminate the so-called belt slack from the safety belt system. So-called pyrotechnic belt tensioners are widespread, in which a compressed gas cartridge is arranged in the housing, which after activation releases a large quantity of gas which is under pressure. This gas acts upon the piston, which then moves in the housing. The movement of the piston is transferred to the belt spool via a pinion engaging into the teeth.
Generally, in such belt tensioners, the problem arises that flow losses occur. At the latest at the moment at which the last tooth of the teeth reaches the pinion, usually (through the opening in the housing in which the pinion is arranged) a large outflow opening is produced for the compressed gas. This reduces the power of the belt tensioner, because the piston is acted upon with decreasing pressure towards the end of its stroke.
The object of the invention consists in further developing a belt tensioner of the type initially mentioned to the effect that a tensioning power is maintained over the entire piston stroke.